Welding method for fabricating large area single crystals and the product thereof

ABSTRACT

A METHOF OF PERMANTLY &#34;WELDING&#34; TOGETHER SLABS OF SEMICONDUCTOR MATERIALS CUT FROM GROWN CRYSTALS SUCH AS, FOR EXAMPLE, GALLIUM ARSENIDE IS DISCLOSED. A THIN FILM OF ALLOYING MATERIAL SUCH AS, FOR EXAMPLE, GERMANIUM IS FORMED ON ONE EDGE OF A SLAB OF SEMICONDUCTOR MATERIAL TO BE WELDED TO THE EDGE OF ANOTHER SLAB OF THE MATERIAL WITH THE ALLOYING MATERIAL BEARING SLAB EDGE AND THE EDGE OF THE SLAB TO BE WELDED THERETO JUXTAPOSED THE RESULTING SANDWICH IS HEATED IN A SUITABLE ATMOSPHERE SUCH AS, FOR EXAMPLE, A METALLIC VAPOR ATMOSPHERE ABOVE THE EUTIC POINT OF THE SYSTEM TO DISSOLVE THE ALLOYING MAMATERIAL IN THIN DISSOLVED LAYER OF THE SEMICONDUCTOR MATERIAL TO FORM A LIQUID SOLUTION WITH A COMPOSITION DETERMINED BY THE PHASE DIAGRAM OOF THE SYSTEM. THE TEMPERATURE OF THE SYSTEM IS THEN LOWERED TO ROOM TEMPERATURE TOENABLE THE LIQUID SOLUTION TO SOLIDIFY AND GROW ONTO THE SEMICONDUCTOR MATERIAL SLABS TO FORM AN EPITAXIAL LAYER WELDING THE TWO SECTIONS TOGETHER.

3,826,700 Patented July 30, 1974 United States Patent 01 lice 3,826,700 WELDING METHOD FOR FABRICATING LARGE AREA. SINGLE CRYSTALS AND THE PRODUCT THEREOF Ting lL. Chu, Dallas, Tex., assignor to Texas Instruments Incorporated and Southern Methodist University, Dallas, Tex.

Filed June 28, 1972, Ser. No. 266,932 Int. Cl. H011 7/38 US. Cl. 148-477 18 Claims ABSTRACT OF THE DISCLOSURE A method of permanently welding together slabs of semiconductor materials cut from grown crystals such as, for example, gallium arsenide is disclosed. A thin film of alloying material such as, for example, germanium is formed on one edge of a slab of semiconductor material to be welded to the edge of another slab of the material. With the alloying material bearing slab edge and the edge of the slab to be welded thereto juxtaposed the resulting sandwich is heated in a suitable atmosphere such as, for example, a metallic vapor atmosphere above the eutectic point of the system to dissolve the alloying mamaterial in a thin dissolved layer of the semiconductor material to form a liquid solution with a composition determined by the phase diagram of the system. The temperature of the system is then lowered to room temperature to enable the liquid solution to solidify and grow onto the semiconductor material slabs to form an epitaxial layer welding the two sections together.

This invention relates to semiconductor materials and more particularly it relates to an alloying technique for joining slabs of semiconductor material to fabricate larger area single crystals.

Materials suitable for infrared windows must have good transmission across the infrared spectrum and maintain the good transmission at elevated temperatures. Semiconductor material which possess these characteristics such as, for example, gallaium arsenide make ideal infrared detector windows. However, such materials have to be grown and they have practical growth size limitations. For example, under presently known techniques for growing gallium arsenide, the maximum growth is about 1 /2 inches in diameter. Thus, for windows of larger dimensions, this growth limitation has precluded the use of semiconductor type materials and in particular gallium arsenide.

llt is therefore an object of this invention to provide a window of semiconductor material such as, for example, gallium arsenide having a size exceeding known growth size limitations for use with infrared systems or the like.

It is another object of the invention to provide a method for combining sections of semiconductor material without either disturbing the ability of the material to transmit across the IR spectrum or to increase the load distortion at high temperatures.

It is still another object of the invention to provide a method for combining slabs of single crystalline semiconductor material to produce large area single crystals.

In accordance with this invention, slabs of semiconductor material are combined to form large single crystals for use, for example, as a window for IR systems by the use of an alloying technique to weld together the slabs or sections. The technique includes forming a suitable alloying material on the surface of one section of the semiconductor material to be joined with the surface of another section of like material; heating the system in a suitable vapor environment for maintaining a desired system equilibrium to a temperature above the eutectic temperature of the alloying material-semiconductor material system to dissolve the alloying material in a thin layer on the semiconductor material to form a liquid solution with a composition determined from the phase diagram of the system. The temperature of the system is then lowered to enable the liquid solution to solidify and grow onto the semiconductor materials to weld the two sections together. Large single crystals, which are suitable for windows, fabricated by this technique have been found to be free of defects such as, for example, point, line or boundary defects which if present would degrade the results of the IR window.

These and other objects and features of the invention will become more readily understood in the following detailed description taken in conjunction with the drawlngs.

FIGS. 1 A-D show the procedural steps utilized in the method embodiment of this invention.

FIG. 2 is a phase diagram for the gallium arsenidegermanium system which is typical of the phase diagrams of the semiconductor materials utilized in the embodiment of the invention.

FIG. 3 is a sectional view of a window for an infrared system housing showing the absence of any detector performance degrading defects.

Referring to the drawings, the fabrication of a large area crystal suitable for an infrared detector housing window constituting an embodiment of the invention includes joining or combining by welding together a plurality of semiconductor material slabs 10 (FIG. 1A) using an alloying technique. Slabs of semiconductor material having good infrared transmission at temperatures of up to about 400 C. such as, for example, gallium arsenide are formed from slices of the semiconductor material. The surfaces of the slabts of semiconductor material to be joined are provided crystal orientations conducive to wetting by an alloying material; a {111} orientation is the preferred orientation. The surfaces to be joined are chemico-mechanically polished by polishing techniques well known to those skilled in the art and which need not be described in further detail. After polishing, a layer of alloying material 12 such as, for example, germanium for gallium arsenide slabs is deposited on the surface of one of the slabs 10 (FIG. 1B). The layer 12 of alloying material may be deposited on the surface by any suitable deposition technique such as the vacuum evaporation or chemical deposition techniques. As both of these techniques are well known to those skilled in the art the details of the techniques are not included. Although germanium is the preferred alloying material for gallium arsenide, it will be understood that any alloying material is suitable if: it does not react chemically with the semiconductor material; it has a lattice parameter and thermal expansion coefitcient substantially that of the semiconductor material so that growth, for example, epitaxial growth can be readily achieved; and the alloying material semiconductors material system has a simple eutectic point at a conveniently low temperature; and it has a low infrared absorption coefficient in the 8 to 14 micron range.

The germanium-gallium arsenide pseudo-binary system is ideal in that germanium: does not react chemically with gallium arsenide; has a lattice parameter of 5.66 A. and a thermal expansion coefiicient of 5.8 C.- while gallium arsenide has a lattice parameter of 5.65 A. and a thermal coefficient of 5.9 10- C.- forms a system with gallium arsenide having an eutectic temperature of 865 C.; and has a low infrared absorption coefiicient at 10 microns. The infrared system wavelength spectrum extends from approximately 0.75 to approximately 1000 microns; however, infrared energy generated by the geography of the earth and its fixtures is of particular interest in the 8 to 14 micron range.

The surface of the slab (section) 10' of the semiconductor material to be joined is then placed in contact with the alloying material layer in a crystallographically parallel position as to the alloying material bearing section (FIG. 10) to form a sandwich 14. Next the sandwich 14 is heated, in a furnace such as, for example, a resistance furnace, to above the eutectic temperature in a vapor atmosphere of the most volatile constituent of the system until an alloying material semiconductor material system equilibrium is reached (FIG. 1D). For a germanium-gallium arsenide system the eutectic point is about 900 C. and the vapor atmosphere is an arsenic atmosphere. At 900 C., for example, the thickness of gallium arsenide dissolved in the vicinity of the metal is approximately one-half that of the germanium layer. Thus, the extent of dissolution of gallium arsenide and the amount of the liquid solution can be controlled by varying the thickness of the germanium layer. As the materials melt, the alloying material mixes with the semiconductor material according to the systems phase diagram. The phase diagram for a germanium-gallium arsenide system is shown in FIG. 2. The system is then re moved from the furnace and allowed to cool at room temperature. The slow cooling of the liquid solution to room temperature solidifies the mixture which grows during solidification into what is considered to be an epitaxial single crystalline layer of alloyed semiconductor material. The epitaxial layer is bonded to the original semiconductor material slabs. In the case of a germaniumgallium arsenide system, the bond is very strong mechanically because of the nearly identical lattice parameter and thermal expansion coefficient of germanium and gallium arsenide.

EXAMPLE Two square gallium arsenide single crystal slabs with faces to be joined of {111} orientation were formed from two gallium arsenide crystals slices having a diameter of 1% inches. Approximately 2 microns of germanium were evaporated onto one face of each crystal, and the two crystals were juxtaposed with the alloying material in between and heated to about 1000 C. in an arsenic atmosphere to dissolve about 4 microns of gallium arsenide. Upon slow cooling to room temperature, the 2 crystals were found to be tightly joined by an epitaxially grown layer of germanium-gallium arsenide and could not be separated mechanically without breakage (FIG. 3). In this fashion any number of gallium arsenide chips can be joined welded together to form, for example, a gallium arsenide window of suitable size for an infrared system.

Various changes can be made in the above constructions and inventions without departing from the scope of the invention as defined by the appended claims.

What is claimed is:

1. A method for joining single crystalline semiconductor materials to yield a large area infrared window single crystal comprising:

forming a layer of alloying material upon a surface of a first slab of single crystalline semiconductor material; :juxtapositioning a corresponding surface of a second slab of single crystalline semiconductor material and the first said slab with the alloying 4. material of the first slab in between to form a sandwich; heating the sandwitch to selectively dissolve portions of the first and second sections of the semiconductor material and alloying material; and cooling the sandwich to room temperature to grow a layer of the alloying material-semiconductor material system welding together the two single crystalline grown material sections.

2. A method according to Claim 1 wherein the slabs of single crystalline semiconductor materials are compounds of gallium and arsenide.

3. A method according to Claim 2 wherein the alloying film formed on the section of single crystalline semiconductor material slab is germanium.

4. A method according to Claim 3 wherein the gallium arsenide-germanium sandwich is heated to a temperature above 900 C.

5. A method according to Claim 4 wherein the gallium arsenide-germanium sandwich is heated in an arsenic atmosphere.

6. A method according to Claim 5 wherein the gallium arsenide-germanium sandwich is heated to dissolve about 4 microns of gallium arsenide to form a pool with the melted germanium metal.

7. A method according to Claim 1 wherein the metal has a lattice parameter and thermal expansion coefficient substantially that of the single crystalline semiconductor material.

8. A method according to Claim 1 wherein the alloying material-single crystalline semiconductor material system has an eutectic temperature of about 865 C.

9. A method according to Claim 1 wherein the alloying material has a low infrared absorption coefficient at 10 microns.

10. A method according to Claim 1 wherein the sandwich is heated in a vapor atmosphere of the most volatile element of the alloy-semiconductor material system.

11. A method according to Claim 1 wherein the faces of the slabs of semiconductor materials to be joined are of an orientation conducive to uniform wetting by the alloying material.

12. A method according to Claim 1 wherein the faces of the semiconductor slabs to be joined are oriented for wetting by the alloying material and are juxtaposed such that they are crystallographically parallel.

13. An infrared window crystal of semiconductor material comprising a plurality of slabs of semiconductor material joined together by a grown weld.

14. A single crystal of semiconductor material according to Claim 13 wherein the grown weld comprises an alloying material-semiconductor material system.

15. A single crystal of semiconductor material according to Claim 13 wherein the alloying material-semiconductor material system of the grown weld comprises a germanium-gallium arsenide system.

16. A single crystal of semiconductor material according to Claim 13 wherein the grown weld has a lattice parameter and thermal expansion cofiicient substantially that of the single crystalline semiconductor material.

17. An infrared window comprising a plurality of gallium arsenide slabs joined together by a germanium weld to form a unitary crystal.

18. A method for joining together slabs of gallium arsenide to form an infrared window comprising:

forming a layer of germanium upon a surface of a first slab of gallium arsenide; juxtapositioning a corresponding surface of a second slab of gallium arsenide and the first slag of gallium arsenide with the layer of germanium between to form a sandwich; heating the sandwich to selectively dissolve the portions of the first and second sections of the semiconductor material and alloying material; and cooling the sandwich to room temperature to form a layer 5 6 of the germanium-gallium arsenide system welding 3,503,125 3/1970 Haberecht 29-476 together the two crystalline grown material sections. 3,375,143 3/1968 Garner et a1 148-477 X 1 3,301,716 1/1967 Kleinknecht 148-1.S References Cited UNITED ST PATENTS 5 GEORGE T. OZAKI, Primary Examiner 3,057,762- 10/ 1962 Gans 148-33 US Cl. XR. 3,520,735 7/1970 Kurata ,148-171 X 3 35 502 11 19 7 Rediker 14 1 X 29194, 195,

2,701,326 2/1955 Pfann a a1. 252-623 E 185352-623 GA 

